The present invention relates to novel coating apparatus for applying multiple superposed coatings to the surface of a moving web, and more particularly to coating apparatus in which layers of coating solutions cascade onto a moving web and are thereafter set or gelled.
Known cascade coating apparatus is capable of applying a plurality of separate coatings onto the surface of a moving web. The coatings are depositied onto the web from downwardly flowing coating solution lamina, superposed one upon another in a layer relationship, and thereafter gelled, or set, to produce an article having a plurality of substantially discrete superposed layers on the web base. Conventionally, the coating solutions have been applied to the web as the web is being supported by a roller, i.e., the web is curved in the direction of travel at the time of application of the coating solutions. The use of rotating rolls as web supports in combination with a stationary coating head apparatus has met with success; however, there are a number of disadvantages associated with such structures. For example, when attempting to coat at relatively high web speeds, particularly with high viscosity coating fluids, there have been such problems as air bubbles being entrapped under the coatings hereby providing nonuniformity of the coatings. Such problems have been minimized by applying a vacuum to the side of the web which is to be coated as the web passes around the rotating support roll. The use of a vacuum box in this position has to some extent stabilized the "bead" of the coating solution which forms at the point of contact of the coating solution with with the web. However, this approach has not been fully satisfactory due to, for example, the rotating backing roll, variations in flatness of the coating head facing the roll defects in the roll surface, film base flatness variations, etc. Because of the denoted problems, very small, e.g., substantially nonexistent, coating gaps are difficult to obtain on production machines without scraping at some point in the operation. Defects caused by the above cannot be tolerated in products such as photographic films in which substantially uniform coatings are necessary in order to achieve consistently good quality image reproduction.
Typical prior art patents relating to known structures of the type mentioned above are U.S. Pat. Nos. 2,761,419; 3,206,323; and 3,220,877. This invention is related to my invention described in U.S. Pat. No. 3,749,053.